Lasers that have output wavelengths that can be tuned over a broad range of wavelengths are utilized in a number of instruments that measure the properties of a sample as a function of wavelength. One class of laser that is utilized in such instruments is based on a broad bandwidth gain chips that are placed in an external cavity that uses a grating as one of the “reflectors” in the external cavity. To tune the output wavelength, the position of the grating is adjusted to provide the desired laser cavity length and the angle of incidence of light on the grating is adjusted such that the grating selectively diffracts light of the desired wavelength back to the gain chip.
If the band of wavelengths that are diffracted back by the grating is sufficiently large, more than one mode of the laser cavity can be excited. In this case, the laser output light can hop between modes in an unpredictable pattern over time periods that are less than the time period over which measurements are being made by the instrument that utilizes the light from the laser. The hopping can be induced by any of a number of non-ideal factors such as changes in temperature. In a pulsed laser, insufficient grating resolution allows more than one laser cavity mode to satisfy the lasing conditions when a current pulse is applied to the gain chip. Although other physical mechanisms such as homogeneous gain broadening only permit one laser cavity mode to lase, the lasing cavity mode could be different from pulse to pulse. In a pulsed laser, the variations in temperature during the pulse can cause mode hopping (intra-pulse mode hopping). In addition, there are laser beam pointing errors associated with laser mode hopping. Because each laser cavity has a different frequency the angle of the diffracted light from the grating is different for each laser cavity mode.
A large number of measurements require tuning laser wavelengths repeatedly and good laser beam pointing stability during each measurement. For example, the background can be removed from the sample signal by measuring the background when the sample is absent. In these measurements, laser wavelength tuning repeatability is very important. Since the mode hopping can occur at random times, such variations cause problems for measurements that require a high degree of tuning repeatability and good laser beam pointing stability in the tuning of the laser wavelength.